rooster fertile question please help
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Most cockerels are sexually mature and able to fertilize eggs at 4-5 months old. A pretty good indicator (not absolute however) that your cockerel is sexually mature is if he's crowing. Crowing is usually an indicator that a cockerel's hormones have kicked in, in which case he should be fertile. Also, when your cockerel starts mounting hens, he's old enough to breed. Even though he's young and won't reach his full size until nearly a year, he's still old enough to breed.
I've already put eggs in my incubator when the rooster was less than 5 months old (about 4 3/4 months) and they hatched. But I'm sure there are differences among different breeds and individual roosters.
I've had cockerels crow as early as 3 weeks, Ok, not really crowing but they sound like they were being strangulated and trying to crow, but I don't think they were fertile that young.
I also have 3 three month old clutch mates making some crowing noises. Some sound better than others. But they certainly aren't breeding.
You can tell if an egg is fertile (but not if it will hatch) by looking for the bull's eye when you break it open.
- Monkey Lookout
I'd say that mating is the best indicator, but as dekel18042 points out, fertile eggs have a small dot in the yolk that is lighter than the rest of the yolk. Google "how to tell if an egg is fertile" and you will see some photos on the subject.